Amoretti, having fallen from the graces of the ecclesiastical order at Parma, was forced to relocate to Milan around 1771. Here he became an active member of the scientific community. He was editor of the first scientific magazine published in Milan under the title—from 1775 until 1777--Scelta di opuscoli interessanti tradotti da varie lingue renamed in 1778 Opuscoli scelti until 1803, and further renamed in 1804 as Nuovi opusculi scelti.

He became a conservator, officially called "Dottori del Collegio Ambrosiano", in 1797 of the Biblioteca Ambrosiana (Ambrosian Library) at Milan which is said to be the first public library in Europe having first opened its door to the public in 1609. It was as conservator at the library that the world of exploration history was turned on its head by this paleographer. James Alexander Robertson wrongly identified him as the "prefect" or officer in charge of the Ambrosiana library, an error repeated by a few who have referred to Amoretti although as far as can be ascertained not one has detected prior to this article that Amoretti is the first to assert the Limasawa=Mazaua equation. Filipino religious historiographer Miguel A. Bernad mistakenly identified Amoretti as curator of the library.

In 1797 Amoretti discovered at the Biblioteca the lost Italian manuscript of Pigafetta on Magellan's voyage, considered by most Magellan scholars as the oldest of four extant manuscripts and the most complete, although there is consensus among paleographic scholars this and all surviving codices are mere copies of an original or originals now deemed forever lost. The three other extant manuscripts are all in French of which two are conserved at the Bibliothèque Nationale, MSS 5650 and 24224, the last, viewed as the most "princely" of all, is conserved at the Beinecke Library of the Yale University Library, in the United States.

Among other things Amoretti modernized the Italian of Pigafetta's text. His edition was the basis for the writings on Magellan's expedition by Jose Toribio Medina and Francis Hill H. Guillemard whose biography of Magellan is still considered the leading work on the Portuguese navigator. Navigation historians and Magellan scholars, among them James Alexander Robertson, Donald D. Brand, and Martin Torodash, fault Amoretti's edition for taking liberties with Pigafetta's text. Robertson accused Amoretti of committing "the sin of editing the precious document, almost beyond recognition." Brand describes the work as "somewhat garbled." Theodore J. Cachey Jr. (The First Voyage Around the World, (1519-1522), An Account of Magellan's Expedition by Antonio Pigafetta, New York: 1995) called Amoretti's edition as having "bowdlerized the text in an effort to 'exposit with the necessary decency the account of some strange customs written by him [Pigafetta] in frank terms which would offend the delicacy and modesty of the reader of good taste."

Even while it is so poorly regarded Amoretti's work has left an enduring geographical puzzle—an invalid geographical assertion—which has only been recently detected.

In two footnotes on pages 66 and 72, Amoretti surmised that Magellan's port—which he named Massana and appears otherwise as Mazaua or Mazzaua in the clear calligraphic writing of the Beinecke-Yale codex, where the Armada de Molucca anchored from March 28 to April 4, 1521—may be the Limassava in a map of the Philippines by French cartographer Jacques N. Bellin.

Bellin's map is a perfect copy of a chart made by Fr. Pedro Murillo Velarde, S.J., of the Philippines in 1734. Murillo's map was such a brilliant and beautiful map many European mapmakers plagiarized it outright. To the credit of Bellin, he cites Murillo as his authority; he corrects Murillo's longitude which followed the erroneous entry of Pigafetta. The French Bellin was hydrographer to the king of France and one of the greatest and most important French cartographers of the mid-18th century. His works were of such excellence as to set a high standard and were widely copied throughout Europe. His map of the Philippines came out the same year that Murillo's map came out.

In any case, Amoretti offers one proof in support of his guess: the latitude of Limassava is at Pigafetta's latitude for Mazaua at 9°40' North. He was mistaken on two counts, Limasawa is at 9°56' North while Mazaua had three latitude readings by three members of the Armada, 9°40' N by Pigafetta, 9°20' N by Francisco Albo, and 9° N by the Genoese Pilot.

Mazaua was the port where Magellan's fleet anchored for one week. It was also the port where 22 years later, Ginés de Mafra, revisited, the only crewmember of the Armada to do so. There are other visits by Spanish and Portuguese during the entire Age of Sail, the last notable one a few months before the 1565 arrival of the Legazpi expedition being a Portuguese squadron that virtually wiped out the entire population of the isle except for one native who was able to hide.

In fact, it was invented in 1667 by a Jesuit historian who had not read any of those accounts. Fr. Francisco Combés, S.J., had read three works that refer to the Mazaua episode: by Giovanni Battista Ramusio, who said the port was "Buthuan", and this Combés adopted; by Antonio de Herrera, who said it was "Mazaua", which Combés rejected; and by Fr. Francisco Colín, S.J., who said the island was Butuan. Colín pointed to another island he called Dimasaua to signify it is not (di is Bisaya for not) the isle where an Easter mass was celebrated. The island is Pigafetta's Gatighan. In the case of Combés, who wrote five years after Colín, he did not adopt Dimasaua because his story does not mention any mass at all.

Amoretti had not read Colín and Combés and was unaware that their Dimasawa and Limasawa were born of ignorance of the true Mazaua episode. That the island whose names they invented pointed to Pigafetta's Gatighan.

Amoretti also had not read the French manuscripts of Pigafetta which described Mazaua, Magellan's lost harbor, as having plenty of gold mines, and the other firsthand accounts by de Mafra, Albo, the Genoese Pilot, and Ayamonte. These uniformly referred to an island with an excellent port. Limasawa has no anchorage.

His claim asserting identity between Limasawa and Mazaua was false from the very basic level of anchorage.

Navigation historians and Magellan scholars who followed in the wake of Amoretti adopted uncritically his Limasawa=Mazaua dictum. Not one raised any question or doubt. Among these were Lord Stanley of Alderley, Jose T. Medina, F.H.H. Guillemard, Andrea da Mosto, Charles McKew Parr, James Alexander Robertson, down to the latest authors like Laurence Bergreen. By the time Robertson came into the picture, Amoretti's guess became a certainty, this without any additional argument or evidence. Mazaua, declared Robertson, "is doubtless the Limasaua of the present day." It was de rigueur for Magellan writers to state that Limasawa was Pigafetta's Mazaua. In 2003 Bergreen broke away from this and completely disregards the name "Mazaua" altogether, he names the port Limasawa without citing Pigafetta's "Mazaua."

Philippine historians and historiographers who have written on Magellan and the Mazaua episode are totally ignorant of Amoretti. This incredible phenomenon may be explained by the practice or non-practice of modern historiography which is marked by a strict if not reverential respect for authorship.

In the Philippines, Amoretti's dictum was completely accepted but his authorship unrecognized if not indeed appropriated. His assertion was reworked in such a manner as to make it look new and original. Instead of saying what Amoretti said that "Limasawa may be Mazaua" Philippine historiographers reframed it as "the site of the first mass" (Mazaua) is not Butuan (Gian Battista Ramusio's version) but Limasawa.

The framework finally blossomed to the classic question, "Where is the site of the first mass, Limasawa or Butuan?" This proposition forces the reader to choose between two false options, an isle that has no anchorage, Limasawa, and a place that is not an isle, Butuan.

Only with the discovery of the Ginés de Mafra account and by tracing the Limasawa=Mazaua dictum to Amoretti was it possible to see that Amoretti's assertion was based on ignorance of all the eyewitness accounts which he had not read and the accounts of Colín and Combés whose invented names did not point to Magellan's port. De Mafra described a port that was in the Genoese Pilot's 9°N; at this location, all the other's testimonies converged, harmonized to create a unified whole and a consistent truth.

The notion Amoretti propounded that Limasawa is Mazaua is based on ignorance of a basic fact: Limasawa has no anchorage as described by the Coast Pilot of 1927 published by the United States Coast and Geodetic Survey states, "Limasawa is fringed by a narrow, steep-to reef off which the water is too deep to afford good anchorage for large vessels. It also rests on a garbled text of Pigafetta by Ramusio. Finally, it comes from Combés's renaming of Pigafetta's Gatighan into Limasawa.

Pigafetta, Antonio. c. 1523. The First Voyage Around the World (1519-1522), by Antonio Pigafetta. Theodore J. Cachey Jr. (ed. based on James Alexander Robertson’s English translation of the Ambrosiana codex as transcribed by Andrea da Mosto), New York: 1995.